We are attempting to enable Screenshots in unit tests. Currently when we do so, browser_ext_tabs_executeScript.js and browser_ext_tabs_insertCSS.js both fail with an extra messageManager being present. 123 INFO TEST-UNEXPECTED-FAIL | browser/components/extensions/test/browser/browser_ext_tabs_executeScript.js | Pending response count - Got 1, expected 0 Stack trace: chrome://mochikit/content/browser-test.js:test_is:928 chrome://mochitests/content/browser/browser/components/extensions/test/browser/browser_ext_tabs_executeScript.js:testExecuteScript:263 Tester_execTest@chrome://mochikit/content/browser-test.js:752:9 Tester.prototype.nextTest</<@chrome://mochikit/content/browser-test.js:672:7 SimpleTest.waitForFocus/waitForFocusInner/focusedOrLoaded/<@chrome://mochikit/content/tests/SimpleTest/SimpleTest.js:791:59 70 INFO TEST-UNEXPECTED-FAIL | browser/components/extensions/test/browser/browser_ext_tabs_insertCSS.js | Pending response count - Got 1, expected 0 Stack trace: chrome://mochikit/content/browser-test.js:test_is:928 chrome://mochitests/content/browser/browser/components/extensions/test/browser/browser_ext_tabs_insertCSS.js:testExecuteScript:112 Tester_execTest@chrome://mochikit/content/browser-test.js:752:9 Tester.prototype.nextTest</<@chrome://mochikit/content/browser-test.js:672:7 SimpleTest.waitForFocus/waitForFocusInner/focusedOrLoaded/<@chrome://mochikit/content/tests/SimpleTest/SimpleTest.js:791:59

In my latest try run with Screenshots 6.6.1 and with it enabled for tests, this seems to have gone away. https://treeherder.mozilla.org/#/jobs?repo=try&revision=ac58b8b3c33146661ffdb796e80cd98bcd19fd03 Not sure what fixed it so WFM.